Viruses with ten-segmented, double-stranded (ds) RNA genomes from the family Reoviridae, genus Orthoreovirus, are believed to replicate in distinctive, cytoplasmic inclusion bodies (Broering (2002) J. Virol. 76:8285; Dales (1963) Proc. Natl. Acad. Sci. USA 50:268; Dales et al. (1965) Virology 25:193; Fields et al. (1971) Virology 43:569; Mayor (1965) J. NCI 35:919; Mbisa et al. (2000) Virology 272:16; Parker et al. (2002) J. Virol. 76:4483; Rhim et al. (1962) Virology 17:342; Sharpe et al. (1982) Virology 120:399; Silverstein et al. (1968) J. Cell Biol. 36:197; Silverstein et al. (1970) Virology 41:564; Spendlove et al. (1964) Cancer Res. 24:1826; Touris-Otero et al. (2004) J. Mol. Biol. 341:361; Touris-Otero et al. (2004) Virology 319:94). These inclusions are commonly called viral factories (Fields et al. supra; Parker et al. supra) or viroplasms (Touris-Otero et al. supra) and are similar to cytoplasmic inclusions formed by other viruses in the same family. In cells infected by rotaviruses or orbiviruses, for example, these structures are called viroplasms (Eichwald et al. (2004) J. Gen. Virol. 85:625; Silvestri et al. (2004) J. Virol. 78:7763) or viral inclusion bodies (Brookes (1993) J. Gen. Virol. 74:525; Theron et al. (1996) Arch. Virol. 141:1143), respectively.
Many viruses sequester their replication machinery within localized structures or surfaces in infected cells: for example, herpes simplex virus (double-stranded (ds) DNA genome, family Herpesviridae) in nuclear inclusions (reviewed in Roizman et al. (2001) Herpes simplex viruses and their replication, pp. 2231-2295. In D. M. Knipe and P. M. Howley (ed.), Fields Virology. Lippincott Williams & Wilkins, Philadelphia), vaccinia virus (dsDNA genome, family Poxyiridae) in cytoplasmic inclusions also called viral factories (reviewed in Moss, B. (2001) Poxyiridae: the viruses and their replication, 2637-2671. In D. M. Knipe and P. M. Howley (ed.), Fields Virology. Lippincott Williams & Wilkins, Philadelphia), brome mosaic virus (single-stranded (ss) RNA genome, family Bromoviridae) on the cytoplasmic face of the endoplasmic reticulum (Restrepo-Hartwig et al. (1996) J. Virol. 70:8908; Schwartz et al. (2002) Mol. Cell. 9:505), and flock house virus (ssRNA genome, family Nodaviridae) on the cytoplasmic face of mitochondria (Miller et al. (2001) J. Virol. 75:11664).
In early studies, reovirus factories were determined to contain fully and partially assembled viral particles, viral proteins, dsRNA, microtubules, and “kinky” filaments proposed to be intermediate filaments, but not membrane-bound structures or ribosomes (Dales (1963) Proc. Natl. Acad. Sci. USA 50:268; Dales et al. (1965) Virology 25:193; Mayor supra; Rhim et al. supra; Sharpe et al. supra; Silverstein et al. (1970) Virology 41:564; Spendlove et al.). The factories have a peculiarly dense consistency that distinguishes them from the adjacent cytoplasm and causes them to appear highly refractile by phase-contrast microscopy. The determinants or features of one or more viral proteins that would make them capable of forming such a matrix are not well understood.